1. The Field of the Invention
The present invention is related to an apparatus and methods for simulating thermal, acceleration, and ignition pressurization loads upon the grain support system of a solid rocket motor during ignition. More particularly, the present invention is related to an antisymmetric test device for use in applying uniaxial tension on an antisymmetric test specimen designed to simulate the grain support system of a solid rocket motor.
2. Technical Background
A typical solid rocket motor essentially consists of a steel casing filled with a solid propellant grain. A grain support system is utilized to affix the grain within the casing. A typical grain support system consists of a layer of insulation bonded between the propellant grain and the casing. The layer of insulation provides a surface to which the propellant grain may be bonded and assists in shielding the casing from the extreme heat generated when the propellant is ignited.
In the manufacture of the solid rocket motor, the layer of insulation is generally bonded to the rocket casing (or to a liner which in turn is bonded to the casing) through a vulcanization process. That is, the insulation is laid up in the case in a "green" state. As it cures, it bonds to the casing. The grain is then cast onto the insulation with the use of a mold to form an initial internal configuration in the propellant grain.
It will be appreciated that maintaining the integrity of the bonds between the insulation and the casing, and between the insulation and the propellant grain, during ignition of the solid rocket motor is critical to the success of the motor.
When a solid rocket motor is ignited, the bonds within the motor are subjected to extreme forces from a variety of sources. As the rocket accelerates, acceleration loads act upon the bonds. During acceleration, the thrust created by the burning propellant acts upon the head end of the motor. Thus, the grain support system holding the propellant within the motor must be sufficiently strong to prevent the unburned propellant from breaking away and falling out of the motor during acceleration. Of greater practical concern, however, the grain support system must also be sufficiently strong to prevent any significant deformation in the grain, as deformation in the propellant grain can adversely impact the firing characteristics of the motor.
Among the other loads acting on the grain support system are ignition pressurization loads. Ignition pressurization loads tend to expand the case, causing the case to move away from the propellant. In response to these and other loads which induce stress in the propellant grain, and generally tend to separate the propellant from the case, stress relief flaps have been incorporated into some solid rocket motors.
Stress-relief flaps essentially comprise a hinge near the head end of the motor between the case and the layer of insulation surrounding the propellant. Thus, when the forces tending to separate the insulation and propellant from the case are sufficiently strong, the flap will open by permitting pivoting about the hinge, thereby permitting limited separation of the case and the insulation while preserving the integrity of the bonds.
Stress-relief flaps are typically constructed of a moldable material having some insulating value. Such materials include Kevlar and polyisoprene. The flaps may be reinforced with long fibers extending along the entire length of the flap to prevent the flap from tearing. These stress-relief flaps are typically designed such that when they open, they do not induce high strains in the propellant at or near the base of the flap. The use of an appropriately designed flap can also reduce the amount of insulation required at the base of the flap, thereby permitting the use of more propellant and improving performance of the motor.
Thermal loads resulting from the unequal thermal expansion of the case and the propellant during ignition load the stress-relief flap in a manner similar to ignition pressurization loads. In a typical solid rocket motor, the thermal expansion of the propellant may be ten times the expansion of the case. By providing a stress-relief flap, a measured degree of separation of the grain and the case is permitted and the stress to the bonds and the remainder of the grain support system due to the thermal loads is relieved.
As can be appreciated from the foregoing, the continued improvement in the design of solid rocket motors requires testing and analysis to develop grain support systems which include improved bonds and which utilize bond reinforcement devices, such as stress-relief flaps, to increase the effective strength of the bonds. One of the principal methods of testing grain support systems is to test fire a solid rocket motor incorporating the system to be tested. Data obtained from the test can be utilized in determining how well the grain support system will perform under actual launch conditions.
One of the principal disadvantages of this testing method is its expense. For example, if it is desired to test propellant grain reinforcement systems in the solid rocket motors used on the space shuttle, the extreme expense in test firing such motors merely to test a wide variety of experimental designs obviously renders such an approach unfeasible.
Also, the nature of the data which can be obtained from such a test is limited. During a test firing it is difficult to monitor the exact load at which the bonds begin to fail. Such monitoring would be accomplished most effectively by visually monitoring the grain support system as the loads are applied. Obviously, this is not possible during the ignition of a rocket motor, even if the test is a static firing of the motor.
In response to the difficulties and expense of testing grain support systems through firing experimental rocket motors, other testing methods have been developed. One method which continues to be used extensively is finite element analysis. In this method, the reaction of bonds and bond-reinforcement devices to stresses are analytically predicted. A primary advantage to the use of finite element analysis is its low cost, as compared with physical testing through firing of the motor.
The use of finite element analysis, however, is limited in its ability to accurately predict the reaction of experimental materials. Finite element analysis is best suited for use when the physical properties of the materials being employed are well known. In the development of solid rocket motors, however, much of the experimental work is focused on employing materials whose physical properties vary. These materials include composite materials utilizing variations of fiber reinforcement. Such materials are not well suited to analysis using finite element methods.
Thus, it would be an advancement in the art to provide an apparatus and method for testing propellant grain support systems which can be effectively utilized to test materials about which physical properties may not be known.
It would be an additional advancement in the art to provide such an apparatus and method which could be used in the testing of propellant grain support systems which could provide a physical test without the extreme cost attendant in preparing a solid rocket motor according to the experimental design and firing the motor.
It would be a further advancement in the art if such an apparatus and method could be provided to test propellant grain support systems whereby bonds and bond reinforcement systems, such as stress-relief flaps, could be visually monitored during the test, thereby obtaining specific data on the location of debonding and deformation.
Such an apparatus and method are disclosed and claimed herein.